I.] PRINCIPLES AND METHODS. 3 



wick is level with the top of the cork. Then glue the prism on the 

 cork, so that by looking sideways through the prism the coloured 

 image or spectrum of the flame of the candle placed at the other end 

 of the piece of wood can be seen. 



We get a band of colour, a spectrum of the candle flame built up 

 of an infinite number of images of the flame produced by the light 

 rays of every colour. But, so far, the spectrum is impure, because the 

 images overlap. We can get rid of this defect by replacing the candle 

 by a needle. 



If we now allow the needle to reflect the light of the candle flame, 

 taking care that the direct light from the candle does not fall upon the 

 face of the prism, we then get a much purer band of colour, because 

 now we have an innumerable multitude of images of the thin needle 

 instead of the broad flame close together. The needle is the equi- 

 valent of the slit of the more complicated spectroscopes used in 

 laboratories. 



FIG. 2. Use of the simple spectroscope. 



We can vary this experiment by gumming two pieces of tin oil 

 with two perfectly straight edges on a piece of glass so that the straight 

 edges are parallel and very near together. In this way we have a 

 slit ; this should be fixed close to the candle and between it and the 

 prism. 



Now the light of the candle is white, and the preceding experiment 

 tells us that such light gives us a band containing all the colours 



B 2 



